Automatic amplification control



June 9, 1942. s. M. BROWN 2,285,895

AUTOMATIC AMPLIFICATION CONTROL Filed June 15, 19:59 Z'Sheets-Shet 1ffilmlll u WI w 3% g :1: f

His Attorney.

, Jun 9, 1942.

AUTOMATIC AMPLIFICATION CONTROL Filed June 13, 1939 2 Sheets-Sheet 2 M,BROWN 2,285,895

6 eorge M. B rown,

Patented June 9, 1942 AUTOMATIC AMPLIFICATION CONTROL George M. Brown,Schenectady, N. Y., minor to mpany, a corporation of General Electric CoNew York Application June 13, 1939, SerialNo. 278,854

6 Claims. (01. Iva-1n) This invention 'relates to electron dischargeamplifier circuits.

It has been a problem in the art of communication by modulated carrierwaves toobtain a maximum amount of modulation at all times in order touse the carrier wave most efllciently. It

. is desirable to amplify signal waves with which a carrieris to bemodulated in such a way that the-average modulation depth approaches 100per cent as nearly as possible whileretaining the fidelity of the signalwaves. The design of my amplifier is such that signal waves areamplified linearly up to some arbitrary amount which modulates thecarrier wave at an arbitrary depth, such as 90 per cent, whilefor-signal waves of greater level than this arbitrary. value-the."

amplification is reduced so that the output of my amplifier increasesvery slowly above this ar- "bitrary value., It is accordingly an objectof my invention to provide an improved and simplified audio amplifier orconventional construction. which is adapted to supply amplified signalenergy to any utilization circuit and at the same time controls theamount of amplification in responseto the relation of the amplified waveto an arbitrary level.

In certain types of amplifiers,such as those used with transmitters offrequency modulated waves, or phonograph recording apparatus and thelike, it is desirable to provide means responsive to peaks of the signalwaves of either polarity to control the amplification of the amplifier.In

the case of frequency modulated waves the avail-- able band width of thereceiver is definitely limited and the frequency modulated wavestransmittecl must not exceed this band width in their frequency changes.In phonographic recording invention; and Fig. 215 a circuit diagramillus- .trating a modification of the circuit of Fig. 1. Referring toFig. l, a suitable source of audio frequency waves is represented byaninpi1ttransformer l0, across the secondary of which a voltage dividingresistance l I is connected. The primary of the transformer It may besuitably supplied with audio frequency waves from a micro phone or aphonograph pick-up, or the like. One end of the resistance ii isconnected to ground and is also connected through a cathode biasresistor I 2 and a by-pass condenser ii to the cathode of a pentagridtube II. The movable contact of the resistance H is connected to the 3first grid of tube It, which serves to amplify voice waves appliedthrough transformer it. The anode of the tube II is connected through aresistor and a resistor IE to a suitable source of operating potential.The second and fourth grids of the-tube II are screen grids and areconnected through a resistor H to a point between the available groovewidth must not be exceeded on either side. Many communication systemshave similar limitations. It'is therefor an additional object of myinvention to provide improved amplifying apparatus continuously adjustedin amount of amplification in response to the'level of peaks ofalternating voltage of either polarity.

resistors 15 and 16 in order to provide the screen grids with theproperbiasing potential. These two screen grids are. connected by a condenserHi to the cathodeoi! tube i4. i9 connects the point between resistors i5and [5 with the cathode of tube It. The iii'th grid of tube I4 isconnected directly to the cathode and acts as a suppressor grid.

Voice waves applied to the first or control grid of the tube H throughthe transformer it produce corresponding variations in anode potentialof the tube i4 due to the variations in voltage drop through resistorl5. These variations in voltage are coupled by a condenser 20 to thecontrol grid of a duplex diode triode'tube 2|. The

cathode of the tube 2| is connected to ground through cathode biasingresistors 22- and 23 in series. A grid resistor is connected from theand 23 to provide proper bias The features of my invention which Ibelieve to be novel are set forth with particularity in the appendedclaims- My invention itself, however, both as to its organization andmethod of operation, together with further obiects and advantagesthereof may best be understood by ref,-

nection with the accompanying drawings in which Fig. 1 is a circuitdiagram illustrating schematically an audio amplifier embodying my anodeof the tube 2lislconnected through a -resistor 26 to a suitable sourceof operating p05 erence to the following description taken in congrid oftube 2| to a point between resistors 22 potential for'the grid. Theresistor 23 has a sliding contactwhich is connected to ground and isadapted to vary the efiective portion ofv the resistor in the cathodecircuit. Resistors 22 and 23 areshunted. by a by-passing condenser 25.;The

tential.

The grid of tube 21, which is supplied with voltage variations bycondenser 20, causes voltage variations on the anode of'tube 2| due tothe voltage drop through the resistor 2C. These anode voltage variationsare coupled through a condenser 21 to theprimary'of a transformerAnother condenser 28; The'other end or the transformer primary isconnected to the-cathode tube 2|. The ends accuses the fact that thesecondary or transformer 6f the. secondary of transformer 23 are 'conpnected respectively to the grids-of apair 0! tubes 29 and 30 arranged inpush-pull relation. The

secondary of the transformer 23 has a center tap connection which isgrounded and isalso pass condenser connected through a resistor 3|shunted by a by- 32 to the "cathodesoi' the tubes 29 and 30 The anodesof the'tubes 29 and are-connected respectivelyto the ends;of the primaryof atransformer 32. A center tap con? nection of the primary oftransformer 33 isconnected to a suitable source ofanjode potential.' Thesecondary of the transformer 33 is adapted may be poled in eitherdirection.

It should also be. noted that, when values of voltage appear on thediode anode. of tube 2|,

suflicient to be rec fi d, that anode is prevented from risin topositive voltages corresponding to oped across resistor such valuesalthough it does follow the voltage wave as it swings in the negativedirection. An effective negative biasing voltage is thus devel- 35and'isapplied through the resistor 36 to the third grid of the tube N.This negative biasing. voltage reduces the gain of tube H rapidly sothat the output from transformer as rises only veryslightly even with ato supply amplified audio. waves to any suitable device. This devicemay, for example, be the modulationstage in a radio transmitter,withwhich my invention is particularly useful.

"A typicalplate modulated transmitter, as illustrated, may include asource .60 of operating potential for-a carrierwave power amplifierdevice great increase in the input to the transformer L0. The criticalvalue at whichthis. reduction in gain begins to occur may be adjusted bymeans of the sliding contact on the variable resistor 23. By movementsistance between the cathode of tube 2| and the ground is variedwith theresult that the positive voltage of the cathode of tube 2| with re- G l.Thenegative terminalof the source is grounded and'the positive'terminalis connected through-the secondary of transformer 33 andthrough a tuned circuit 82 to the anode of the device il, whosecathodeis grounded. A earrier wave source 63 impresses'acarrler wave upon thedevice Six-to be amplified therethrough byan amount governed by theoperating potential supplied through transformer 33'. The amplifiedcarrier wave in the tuned circuit 52 'is' radiated from an antenna 64.

The anode of tube 29 is coupled through a condenser 34 to a conductiveimpedance 35,

which is'shown or convenjeneeas a. pure resistanc'e' This resistance, isgrounded at its other end. Volta e variations of this anode thereforespect to ground may be adjusted; For any particular adjustment of thepositive voltage of this cathode thereduction in gain of the tube [4begins to occur when the level of the peaks of voltage on the diodeanode is equal to the positive voltage of the cethode of tube 2|.

The proper adjustment of my amplifier for use me radio transmitter issuch that the output of transformer-33 rises linearly in accordance withthe-input to transformer up to a critical e I 33 is sumcient per cent.,Any other. suitable .depth may be appear. across'the resistance- 35 Adiode anode of the'tube MT is. connected to a point between condenserand resistance 35.' v

The cathode of tube 2| remains at a substantially constant positivepotential with respect to* ground. due to resistor 22 and the variableresistor 23. The diode anode o f'the tube 2| there-- tore allows currenttopass only when its voltage through a. conductive impedance 33, which.isv "shown as a pure resistance, to the thirdgrid of rises to positivevalues greater than the substantially constant voltage existing-acrossres istor' 22- and variable resistor 23.

Whenever such high positive values'of voltage Q are rectified throughthe'diode anode, a continuousvoltage is developed across resistor 35.

The voltage across this resistor 35 is supplied the 'tube 14. This thirdgrid s connected through a condenser 31 to ground' This con- *-denseris'ot sufficiently large value that voltage variations which occur atrelatively high, speed 'do not affect the third gridoi the tube II. The

mentary transients.

value at which the output from the transformer V to modulate the carrierwave with a' predetermined depth, such, for exam le, as 90 chosen asdesired. ,The' operation of my ampliher then prevents any great rise inoutput from transformer 33 above thisvalue and accordingly It has beenfound that some music and most speech, when translated into electricwave en-. F 50 has greater peak amplitudes of one polarity than of theother. Sincethe phenomenon called over-modulation withthe accompany gradiaresistor 36 in conjunction with .the condenser- 31, in fact, .formsa filter; which has a'time contlon or spurious signais is dueprincipally to fiat-' tening of the carrier envelope on the zero axisrather than by extension of the maximum carvrier amplitude beyond'thelevel at twice the amplitude of the carrier when unmodulated, it isdesirable that such higher peaks-of a signal wave produce the positivemodulation peaks or maximum carrier amplitudes and that. the smallerpeaks of opposite polarity produce the negative modulation peaks of thecarrier. The provision of such operation allows more energy to beradistant. of the order 'oI-a fifth of a second. This time constant mustbe such as to prevent the automatic control embodied inmy invention from'taking effect so quickly as to smooth out variations of the speechwave. However, the filter ated for a given signal wave, assuming maximumpermissible modulation, with a corresponding increase in emciency. It isdesirable, therefore, to

- secure such'operation, to pole the-output from circuit-must respondwitli suflicient speed to bias the third grid of tube on the occurrenceof peaks in the voice wave.

It should be noted that positive peaks of voltage on the anode of tube29 may correspond transformer v33 toinsure that the peaks of higherpolarity produce positive modulation peaks. It

is, of couIse first desirable to polethe input to I transformer ",jromthe microphone or the like,

- so thatv therednction of amplification either to" maximum or minimumamplitudes'of thecarrier, or 'in other. words, to either positiveornegative modulation peaks. This is due to initiated in t0 e l w r. peaksofthe s gnalwavesw Since there is an of this contact the amount ofreunavoidable from one side of. thepushrpnllcircuit includingtransformer It as in Fig. l.

'sistor 24 is connected directly to ground.

tubes 29 and 30 to the third grid of tube l4 through condenser 34 andresistor 38, there may be some ,feedback which shouldbe minimized bycondenser 31. This feedback may be further reduced by includingadditional low pass filters in cascade with the one formed by resistor33 and the capacitor 31. In the design of this particular circuit, oneshould connect the, condenser 34 to that anode of the push-pull stagewhich creates degeneration, in case there is any remaining feedback.This is desirable to obviate any possibility of oscillation.

A very convenient way of minimizing feedback is by the use of a bridgecircuit in which audio voltages of o'pposite'phase are fed from therespective tubes 29 and 30 to the third grid of tube I4. In Fig. 1 acondenser 41, a-resistance 48,

and a switch 49 are connected in series between" the anode of tube 39and the third grid of tube l4. The condenser 41 should be substantiallyidentical with condenser 34 and resistance 48 with resistance 36. Withthe switch 49 open the operation of the circuit is as has beendescribed. When the switch 49 is closed an audio voltage is supplied tothe third grid of tube l4 in such condenser 34 and resistance 36 fromthe anode of tube 29 is substantially canceled.

A switch 50 and a resistance 9| are connected in series between groundand a point between condenser 41 and. resistance 43. The resistance 5|should be substantially identical with resist: ance 35. completecancellation of the audio voltage feedback to the third grid of tube I4is attained, since 1 the bridge is more nearly perfectly balanced.

In certain cases, for example, in frequency modulation or inphonographic recording where it is desirable tolimit amplification inresponse to signal peaks of either polarity, it maybe convenient to biasthe third grid of tube 14 with a bias voltage derived from signal peaksof either polarity which are greater than a ilxed voltage. In such casesa switch 52 connecting the other diode anode of the duplex tube 2| to-apoint between the switch 50 and theresistance 5| may be closed. Suchaconnection, of course, retains the advantage of' the bridge circuitdescribed. This other diode anode develops a biasing voltage on thethird grid of tube I4 in the same way as the first diode anode with itscoupling condenser 34 and resistances and 36. with separate sources ofbias voltage may be substituted for the diodes in tube 2|. This ar-.rangement maybe better understood by considering the "principles ofoperation of the circuit illustrated .by Fig. 2.

Referring to Fig. 2, certain parts of the 'circuit illustrated therebyare identical with those shown in Fig. 1, and like parts have beengivenphase that the audio voltage fed back through i voltage variationsby condenser 20, causes similar voltage variations on the anode of tube38 due to the voltage drop through resistor 26 which is conected betweenthe anode of device 33 and a suitable source of operating potential. Thevoltage variations across resistor 26 are coupled as in Fig. 1 to a pairof tubes 29 and 30 arranged in push-pull relation. The anodes of thesetubes are connected to the output transformer 33.

A pair of'voltage dividing resistances as and 40 are connected across asuitable source of direct current. The sliding contact of the resistance39 is connected through a conductive impedance 41, which is shown forconvenience as a pure resistance, to the cathode of. a diode element ofa tube 43. Similarly the sliding contact of the resistance 43 isconnected through a conductive impedance 42, also shown as a resistor;to the cathode of a second diode element of the tube 43. The cathode ofthe first diode is connected through a condenser 44 to the anode of tube30. The cathode of' the other diode is connected through a similarcoupling condenser 45 to the anode of tube 29.

The anodes of these two diodes are connected together and to groundthrough a resistor 46.

, These anodes are also connected through resistor When the switch isclosed a more 36 to the third grid of tube H, which is coupled by thecondenser 31 toground. 1

In operation, under the condition when no signal is being amplified, thecathodes oithe tube 43 are maintained at positive. potentials withrespect to ground by amountsequal to the voltage from-the slidingcontact of the respective resistances 39 and--40 to ground. The'anodesof tube 4.3 are maintained at ground potential by resistor 46. a

.'When signals are amplified by the arrangement, the voltage variationsof the anodes of tubes 29 and.30 are impressed on the. cathodes oftube.43. These voltage variations due to signals'make the cathodes 0!tube 43 less positive during certain portions .of the signal wave.

When these cathodes become negative due to a signal of greater levelthan that. necessary to overcome the respective positive bias voltagesproduced by resistances 39 and 40, the tube 43 Separate diodes 1 passescurrent and the anodes ,of tube-43 become negative with respect toground. This produces a similar negative voltage on the third grid oftube 14 and reduces the amplification thereof in a manner like that ofthe circuit of Fig. 1.

It shouldbe noted particularly that tube 29 amplifies signal voltageswings of one polarity and tube 30 those of the other.- Hence, if thesignal peaks of either polarity increase more than predeterminedamounts, the tube 43 passes current in the corresponding diode elementand the amplification of tube I4 is reduced. It should also be notedthat the predetermined levels at which the two diode elements passcurrent may I be separately adiusted by the sliding contacts of shuntedby by-passing condenser 25 connects the a cathode of tube 33 to ground.It should be noted that resistor 23 is omitted, and that the gridrethe'two' resistances 39, and 40. They are preferably adjusted inproportion to the magnitude of alternating voltage on-thg" respectiveanodes of tubes 29 and 30 for the particular type Of signal beingamplified.

It may be desirable only to limit the signal peaks of either polarity tothe sam value. In

such case, the resistance 40 may be omittedand the resistor 42connectedtothe sliding contact of resistor 39. It is, of course,.obvioii that one diode element may be omitted withiits corresponding resistor,condenser and'voltage "divid The grid of tube as, which issupplied with

